II. 



A SURVEY or THE SPIRAL JNEIiULA MESSIER 33 BY 

 MEANS OF PHOTOGRAPHS TAKEN EY DR. ISAAC 

 ROBERTS, E.R.S. 



Br J. L. E. DREYER, Ph.D. 



Read April 25. Ordered for publication May 11. Published Sept. 6, 1904, 



Plates I. and II. 



The great nebula known as Messier No. 33 is situated in the con- 

 stellation Triangulum, and was first noticed by Messier in 1764. 

 He describes it as having a whitish light, almost equable throughout, 

 but a little brighter over two-thirds of the diameter, containing no 

 stars, and being 15' in diameter. William Herschel mentions it in 

 his papers in the Philosophical Transactions for 1785 and 1818. In 

 the former he says that its breadth from north to south is probably 

 not less than 30', but that it fades away so gradually that its limits 

 are difficult to fix. In the paper of 1818 he mentions that when 

 seen in his seven-foot reflector it had a mottled aspect, but that a ten- 

 foot telescope resolved it into stars, " the smallest points imaginable." 

 In reality, :he nebula is not "resolvable," that is to say, it is not a 

 cluster of stars ; but Herschel doubtless was able to perceive many of 

 the minute stars scattered over the nebulosity, as well as some of the 

 condensations or " knots," and he concluded, therefore, that the nebula 

 was resolvable. North of its centre, but situated on one of its 

 branches, is a small nebula (III. 150 = ^ 133), looking like an ill- 

 defined star, "almost planetary," according to d' Arrest, observing in 

 1864, which description has been confirmed by the spectroscope, since 

 the spectrum is a gaseous one. 



In a small telescope, and even in a moderate-sized one, M. 33 does 

 not look an interesting object. It only occurs once in Sir John 

 Herschel's series of observations (Phil. Trans., 1833); he remarks 

 that " the diffused neb. extends 15' south, and as much nearly north ; 

 it has irregularities of light, and even feeble subordinate nuclei, and 



A 2 



